1. Field of the Invention
This invention relates to electrically conducting compositions, useful as electronic and optical materials, comprising a solid polymer polyphenylene and as the doping agent to confer electrical conductivity, an electron donor or acceptor, or mixture thereof.
2. Brief Description of the Prior Art
The electronics industry is continuously searching for new and improved materials for fabricating electronic components. Likewise, the plastics industry is in need of materials which have the property advantages of conventional organic polymers, but which are also electrically conducting. There is a special need in these industries for obtaining conducting materials exhibiting direct current conductivities of at least about 10.sup.-3 ohm.sup.-1 cm.sup.-1, and preferably greater than one ohm.sup.-1 cm.sup.-1, which also have the desired processibility, mechanical properties, low cost, and low density (i.e., light weight) characteristics of carbon backbone organic polymers.
Conductivities referred to herein, unless otherwise specified, are as measured at room temperature.
Carbon backbone polymers, possessing conductive properties, can be prepared by pyrolyzing or graphitizing organic polymers, e.g. via dehydrogenation at elevated temperatures. Such resulting conductivity is most likely due to the formation of electrically conducting graphitic structures. However, such pyrolyzed or graphitized polymers are difficult to prepare in a controlled fashion, and accordingly are likely to exhibit undesirable variation in electrical properties; and are not conveniently processible.
The only non-pyrolyzed non-graphitic carbon backbone organic materials known to us with conductivities as high as 10.sup.-3 ohm.sup.-1 cm.sup.-1 are complexes of unsubstituted polyacetylene, --HC.dbd.CH--x, with specific electron donor or acceptor agents.
Researchers have tried for over twenty years to obtain highly conducting complexes of carbon backbone polymers. Recent efforts in the field to discover substituted polyacetylenes which also form highly conducting complexes, comparable to those of unsubstituted polyacetylene, have not been successful.
Complexes of uncrosslinked polyacetylene with iodine, iodine chloride, iodine bromide, sodium, and arsenic pentafluoride having conductivities ranging from that of the undoped polymers to between 50 to 560 ohm.sup.-1 cm.sup.-1 at 25.degree. C. have been described in J. Am. Chem. Soc. 100, 1013 (1978). Also described therein are complexes of polyacetylene with hydrogen bromide, chlorine, and bromine having conductivities of up to at least between 7.times.10.sup.-4 and 0.5 ohm.sup.-1 cm.sup.-1, where the highest conductivity in this range is for the bromine complex. L. R. Anderson, G. P. Pez, and S. L. Hsu in J. Chem. Soc., Chemical Communications pp. 1066-1067 (1978) describe bis(fluorosulfuryl) peroxide, FSO.sub.2 OOSO.sub.2 F, as a dopant for polyacetylene to produce a composition having a room temperature conductivity of about 700 ohm.sup.-1 cm.sup.-1. Also, the reference of J. Chem. Soc. Chem. Comm. 1979, p. 594-5 reports that poly-acetylene films electrochemically doped with aqueous KI solution or, Bu.sub.4 NClO.sub.4 solution in methanylene chloride provide materials having room temperature conductivities of up to 970 ohm.sup.-1 cm.sup.-1. The possible application of polyacetylene complexes as electronic devices is described in App. Phys. Lett. 33, pp. 18-20 (1978).
Although such polyacetylene complexes are useful as electronic materials, they possess the disadvantages of environmental and thermal instability of the matrix polyacetylene. For example, even in the absence of oxygen, cis-polyacetylene transforms to trans-polyacetylene at a rate of a few percent a day at room temperature, which could result in a variation of electrical properties. Additionally, both isomers of polyacetylene readily react with oxygen, thereby changing the electronic properties of either doped or undoped polymer. The infusibility and insolubility of high molecular weight polyacetylene militates against melt or solution processing and the thermal reactivity in the form of crosslinking reactions also restricts or precludes the possibility of molding this polymer into shaped articles.
Certain charge transfer complexes of poly(p-phenylene) have been reported, none having conductivity as high as 10.sup.-3 ohm.sup.-1 cm.sup.-1 (Organic Semiconducting Polymers, J. E. Katon, Ed., 1968, p. 176).
In contrast to polyacetylene, poly(p-phenylene) although a conjugated polymer, has high thermal and oxidative stability. This polymer is stable at temperatures as high as 400.degree. C. in air and 550.degree. C. in inert atmospheres. Furthermore, poly (p-phenylene) exhibits exceptional resistance to radiation damage, as described in J. Polym. Sci. A3, pp. 4297-4298 (1965). Another advantage of poly(p-phenylene) is that objects having tensile strengths as high as 34,000 kPa (5000 psi) can be molded from this polymer using powder-metallurgical forming techniques without chemical degradation of the polymer, as described in J. Applied Polym. Sci. 22 pp. 1955-1969 (1978). Previous efforts to obtain poly(p-phenylene) complexes having conductivities as high as 10.sup.-3 ohm.sup.-1 cm.sup.-1 have been unsuccessful. The highest reported room temperature conductivity is 4.times.10.sup.-5 ohm.sup.-1 cm.sup.-1, which was obtained for a poly(p-phenylene) complex with iodine, as described in Polymer Preprints 4, pp. 208-212 (1963). The temperature dependence of conductivity, .sigma., is given by the expression .sigma.=.sigma..sub.0 e.sup.-E/kT where .sigma..sub.0 is a material constant, k is Bolzmann's constant (8.6.times.10.sup.-5 eV/degree), T is temperature in .degree.K, and E is the activation energy for conductivity. E is reported by these authors to have a value of 0.87 eV between room temperature and -100.degree. C. By contrast, a much smaller temperature dependence is normally expected for a material to be a useful semiconductor. The above reference also describes the formation of a complex between poly(pphenylene) and tetracyanoethylene. However, in this case the observed conductivity at room temperature (10.sup.-11 ohm.sup.-1 cm.sup.-1) is even lower than that observed for the iodine complex.